This invention relates generally to radiation-hardened integrated memory circuits or embedded memory circuits, and, more particularly, to a novel technique for permanently programming a pattern into the memory.
Radiation is absorbed in materials by two main mechanisms. The primary mechanism is by ionization in which electron-hole pairs are created. If the energy of the radiation is greater than the energy required to create an electron-hole pair, ionization can occur. The energy required to form an electron-hole pair in silicon is 3.6 eV. For each RAD in silicon, approximately 4.0 E13 electron-hole pairs are formed per cubic centimeter. The energy required to form an electron-hole pair in silicon dioxide is 17 eV. Because of the difference in ionization energy, approximately 4.7-times more pairs are formed in silicon than in silicon dioxide for a given dose.
The secondary mechanism is by damage to the lattice structure of the material. Typically, lattice damage occurs due to radiation in the form of high energy neutrons, protons and heavy ions. The particle must have enough energy to break multiple bonds and to move the lattice atom away from its original site.
The interaction of ionizing radiation with materials is fairly complex. But a basic understanding of the effect of the “total dose” radiation on MOS electronics can be gained by examining ionization effects, i.e. the generation of electron-hole pairs the gate and field oxides of an MOS transistor.
Ionizing events occur when electrons in the semiconductor's valence band are raised to the conduction band. A fraction of the electron-hole pairs will undergo geminate (or initial) recombination and cause no damage. Geminate recombination decreases as the electric field increases, and the electrons and holes that survive it are free to diffuse and drift within the oxide where they will be swept from the insulator, recombine, or be trapped.
The trapped charge, particularly the trapped holes, causes much of the degradation of device parameters by radiation exposure. In device-quality silicon dioxide, the electrons move freely out of the oxide whereas the holes are more likely to become trapped. The trapped holes generate space-charge fields in the underlying silicon substrate, resulting in negative shifts in the threshold voltage.
As the threshold voltage shifts negatively, an N-channel transistor biased in the off-state lets more and more current pass. If enough holes are trapped, the N-channel transistor will remain fully conducting even with zero applied gate bias, transforming an enhancement-mode device into a depletion-mode device.
A cross-section of an MOS transistor is shown in FIGS. 1-4. FIG. 1 shows the transistor before a radiation burst. The transistor includes a silicon substrate 10, N-type source/drain regions 12, an oxide gate region 14A, and a polysilicon gate 16. In FIG. 2 numerous electron-hole pairs generated in the silicon dioxide gate 14B are shown immediately after a radiation burst. In FIG. 3 the holes remaining in the silicon dioxide gate 14C are shown after electron transport. In FIG. 4 the remaining trapped final charge is shown in silicon dioxide gate 14D.
While the generation of trapped holes in an N-channel transistor due to total dose radiation is normally an undesirable effect that must be overcome in radiation hardened circuits, what is desired is a technique for programming an integrated circuit having a memory or embedded memory portion that uses the effect of ionizing radiation to full advantage.